Environmental Engineering Reference
In-Depth Information
FIGURE 9.80
Near-vertical slope in 40-year-old railroad cut standing stable in amphibolite gneiss because of vertical jointing.
(Tres Ranchos, Goias, Brazil). Compare with
Figure 9.81.
right of the photo, shown in
Figure 9.84.
The recommendation was to cut into the landward
slope and relocate the roadway away from the sea cliff along this short stretch. The very
costly alternative was to relocate the roadway inland around and over the coastal mountain.
The major cause of instability in colluvial soil slopes is illustrated in
Figure 9.85.
The
slide debris impedes drainage at the toe and causes an increase in pore-water pressures in
(b) over those in (a). The sketch also illustrates the importance of placing piezometers at
different depths because of pressure variations. Conditions in (b) apply also to the case of
a sidehill embankment for which a free-draining blanket beneath the fill would be neces-
sary to provide stability.
Slope Geometry
General
The significant elements of slope geometry are inclination, height, and form. Aspects of
inclination and height, as they relate to a particular point along a slope, are described in
Section 9.1.4.
This section is more concerned with the form and other characteristics of
an entire slope as they affect seepage and runoff, which can be dispersed by the geomet-
rical configuration of the slope or can be concentrated. The difference influences slope
stability.
The examples given are intended to illustrate the
importance of considering the topography
of an entire slope
during roadway planning and design, not only the immediate cut or fill
area.
Topographic Expression
In both natural and cut slopes, the topographic expression has a strong influence on where
failure may occur since landform provides the natural control over rainfall infiltration and
runoff when geologic factors are constant. In
Figure 9.86,
runoff is directed away from the
